A non-synchronized distributed (ad hoc) packet radio network is a collection of wireless mobile devices forming a temporary network without the aid of any established infrastructure or centralized administration. These networks imply the absence of a central node or controller. There might exist a point where an ad hoc network is connected to a fixed network although this is not necessary. The situations where ad hoc networking is unavoidable include, but are not limited to, the following cases: a class of students exchanging files with their laptops or other portable wireless devices, business partners exchanging data at an airport or a team of rescue workers in the field trying to communicate with each other and the rest of the world.
In ad hoc packet radio networks, it is difficult to maintain a global clock due to lack of, for example, a periodic beacon. In order to transmit delay-sensitive traffic (jitter-sensitive traffic), a reservation mechanism is required. Reservations would imply sending packets equally spaced in time, i.e., T.sub.frame apart. Due to a limited accuracy of clocks in the nodes, the packets will have a tendency to overlap and become lost.
Centralized wireless network architectures in most cases have access points through which the mobile wireless users communicate between each other or with other users on the fixed (wired) network. The access points (or base stations) are in the position to provide synchronization (slot timing) to the mobile users through the use of, for example, periodic beacons. In mobile ad hoc networks, since there is no central node, obtaining and maintaining slot timing among distributed users is a more difficult problem. Having a common slot reference is a desired feature since it usually means improved throughput performance. Another desired feature is the ability to reserve a portion of the channel (i.e., one or more time slots) and use them periodically. This implies the existence of a frame period which is known to all participating wireless devices.
A representative protocol, which uses a frame structure, is the Reservation ALOHA random access scheme. In Reservation ALOHA the time horizon is divided into equal size frames and the frames are divided into equal numbers of time slots. In Reservation ALOHA, packets are sent only at the beginning of a slot. In an ad hoc network, which employs a channel access scheme similar to Reservation ALOHA (i.e., time division multiple access-like with contention), it is difficult to ensure that all nodes are slot synchronized. After a node obtains a position in the frame and continues to transmit periodically, it is possible that another packet will start to overlap due to inaccurate clocks at each node. This effect does not occur in slotted systems since the packets are confined to a single slot but will occur in ad hoc networks. In this document, we address a method which alleviates the problem of packet overlap created by inaccuracies of the clocks at each node in ad hoc networks.
A preferred embodiment of the invention is now described, by way of example only, with reference to the drawings.